\documentclass[letterpaper,10pt]{article}
\usepackage{graphicx}
\begin{document}

\begin{figure}
    \centering
    \includegraphics[scale=1]{merkle.1.mps}
    \caption{merkle.1.mps}
\end{figure}

\begin{figure}
    \centering
    \includegraphics[scale=1]{merkle.2.mps}
    \caption{merkle.2.mps}
\end{figure}

\end{document}
